This invention relates in general to generator cooling and lubrication systems, and more particularly, to aircraft generator, auxiliary pump mode, cooling-oil lubrication for generator units, operating during an extended interim without main power output from a failed generator unit, such as when primary pump cooling and lubricating components have insufficient pressure or when too little cooling-oil is present to function normally.
Aviation safety and dependability necessitates redundancy of electrical power generators, as well as, delayed replacement of same so that electrical power continues to be supplied to all vital functions, such as lights, fly-by-wire controls and computers operating without interruption, during relevant time periods, such as when crossing oceans and wilderness regions. Each engine of a multi-engined aircraft can be used to power a generator system with each generator system being capable of fulfilling the entire electrical power output needs for the aircraft, the power output being produced by the main field winding power generating section. A permanent magnet generator on the same rotor shaft as for the main power produces an auxiliary power output which is normally used by the control section of the generator when it is fully operational. As the number of engines declines, reliability must be increased, although, an auxiliary engine and generator may also be used for meeting the aircraft's electrical power needs. This is crucial if there is an emergency with no other remaining back-up propulsion power paired operative generator systems, but an auxiliary pair adds weight which is not without penalty.
Aircraft generator lubrication systems have heretofore comprised primary pump, multiple scavenging sections which circulate a cooing-oil fluid through a casing and return a predetermined quantity of the fluid lubricant to a central reservoir which serves combined drive and generator units in an integrated fashion. Intermediate filtering and cooling operations are performed on the cooling-oil lubricant in order to siphon-off contaminants and provide for a stable operating temperature while the main power field section of the generator unit is providing the operating load requirements for output electrical power.
A type of generator system which utilizes a constant speed drive transmission which interfaces with the propulsion engine, is known from U.S. Pat. No. 4,284,913, "Cooling Arrangement for and Integrated Drive-Generator System", and it is assigned to the same assignee as the present invention. A constant speed output shaft provides constant frequency electrical power from the generator system. A pair of scavenging pumps for the lubrication coolant fluid circulation is provided, with one pump located in the drive unit and another pump located in the generator unit, with a barrier breather therebetween. This refines the prior art constant speed generator arrangements which has the disposition of both pumps being located within the casing of a constant speed drive unit. This type of generator system, however, still presents excess weight that is introduced by the constant speed drive unit, although variable speed generator system (VSCF) significantly reduce this problem.
An aircraft VSCF electrical power generation system according to U.S. Pat. No. 4,554,501, entitled "Variable Speed Constant Frequency Power System With Auxiliary DC Output" is assigned to the same assignee as the present invention. It takes advantage of the inherent capabilities of a DC-link VSCF system to provide backup auxiliary DC power capability with little weight penalty. This type of generator, however, does not usually incorporate apparatus for mechanically disconnecting the rotating generator from the propulsion engine coupling when the generator has a pump failure or when there is a loss of cooling-oil fluid which renders the generator incapable of producing electrical power output. A variable speed generator is described in U.S. Pat. No. 4,851,723 entitled "Coolant Pump System for Variable Speed Generators", which is also assigned to the present assignee, in which a variable displacement pump is mounted within the generator housing and is mechanically coupled to the rotor such that the pump is driven at a speed proportional to the rotating speed of the generator rotor.
A faulty generator continues to rotate until it is disengaged which is usually when either the shear section for the rotating shaft fails or until it is feasible for the maintenance crew to remove the generator from the airplane engine. This means that it must be capable of rotating in a fault tolerant mode until it can be repaired or replaced with an operative spare unit after the flight is concluded and when a spare unit is available. A constant speed drive with an associated generator which can be disengaged, nearly doubles the number of the generator spares required because specialized maintenance tools are required to service the constant speed drive which results in maintenance time for the constant speed drive that is roughly double that for the VSCF generator.
A U.S. application Ser. No. 07/493,170, filed on Mar. 14, 1990, and now allowed, is entitled "Generator Auxiliary Mode Lubrication System and Method" and discloses a generator system adaptive, cooling-oil lubrication system and method of operating a generator system after a failure has occurred in the primary cooling and lubricating system. The generator system and components for fault mode rely on gravity feed and distribution from an auxiliary reservoir to the critical rotating components. Return of the cooing oil lubricant is by the spinning action of the non-power producing rotor of the generating unit in order to recirculate the cooling-oil fluid back to the auxiliary reservoir. This referenced allowed application Ser. No. 07/493,170 of mine is also assigned to the same assignee as the present application and is incorporated by reference into the present application, especially for its schematic showing of an exemplary VSCF generator system.
Aside from the major goal of obtaining continuous back-up or emergency power for fly-by-wire controls, computer driven displays of vital information, and lights for the aircraft, it is dependability of commercial service which sets the goals for positive pressure circulatory lubrication to be applied to the bearings of a faulty generator. Heat removal is also needed for power losses extracted from a reserve of cooling-oil lubricant that must be kept in circulation for at least three hours after a malfunction of the lubrication system, otherwise a permanent damage result is anticipated.
The commercial operator flying an aircraft with a faulty generator is fulfilling the power output requirements as long as there are backup generators aboard the flyable system, thereby operating with a minimal impact. Every time there is a pump failure or loss of the cooling-oil fluid by breakage or interruption of an oil line or passage in an electrical power generator, the complication to the air travel system is potentially immediate and may have consequential affects in both the short run and the long run.